Metamaterials are defined as artificial materials that are engineered to have properties that are not found in nature, and that are not necessarily possessed by their constituent parts alone. In this sense, metamaterials are assemblies of multiple individual elements or unit cells, and they may be on any scale, from nano to bulk.
Metamaterials offer tremendous potential in a wide range of applications, including, but not limited to, negative refraction, wideband antennas near metal, flat lenses, and cloaking Although there has been considerable progress in passive metamaterials, the bandwidth of these devices remains limited by the resonant behavior of the fundamental particles or unit cells comprising the metamaterials. In contrast, non-Foster circuit elements offer the possibility of achieving performance capabilities well beyond the reach of passive components. As is well known to those of ordinary skill in the art, non-Foster circuit elements are those that do not obey Foster's theorem. A complete wideband double-negative metamaterial design has remained elusive, but is provided by the present invention through the use of non-Foster circuit elements. As is also well known to those of ordinary skill in the art, non-Foster circuit elements can be constructed from arrangements of capacitors, inductors, and active devices, such as Linvill circuits, current conveyors, cross-coupled transistors, tunnel diodes, etc.
The closest known art (although not necessarily pre-dating the present invention) is that of Colburn et al. (U.S. Patent Application Publication No. 2012/0256811). Colburn et al. provide:                A tunable impedance surface, the tunable surface including a plurality of elements disposed in a two dimensional array; and an arrangement of variable negative reactance circuits for controllably varying negative reactance between at least selected ones of adjacent elements in the aforementioned two dimensional array.        
The tunable impedance surface of Colburn et al., however, suffers from several significant shortcomings, including, but not limited to: the fact that it is inherently limited to a two-dimensional (2-D) surface, rather than a three-dimensional (3-D) volume; its requirement for a ground plane; and the fact that it only addresses 2-D negative inductance methods, rather than 3-D negative permittivity methods, negative permeability methods, and double-negative metamaterials that exhibit simultaneous negative permittivity and negative permeability. Further, the tunable impedance surface of Colburn et al. considers the stability of non-Foster circuits, but does not consider a metamaterial design wherein a negative capacitive element or negative inductive element is combined with a positive capacitive element or positive inductive element, resulting in a stable element with a net positive inductance or net positive capacitance.